Scientific balloon platforms have been developed to carry payloads to the upper limits of the atmosphere to observe and record a variety of phenomena. Design, analysis techniques, and film quality have steadily improved such that balloon flights are now performed on a relatively routine basis. Scientists, however, need systems that can carry heavier payloads to higher altitudes for longer durations. This requires balloons which test state-of-the-art film design and production. The degraded reliability of heavy lift systems is well known and emphasizes the need for a more precise approach to balloon design and manufacturing.
The most widely used scientific balloon system is the natural shaped balloon made of thin, balanced, polyethylene film. Unfortunately, balloon strain cannot be predicted accurately by closed form analytical methods. Although balloon stresses are not usually of sufficient magnitude to cause failure of the balloon film, they do cause cracks to propagate and provide a state conducive to amplification of any flaws or manufacturing defects.
Flight testing of balloons has been used successfully to obtain data on atmospheric properties, gas and skin temperatures, radiant flux, and pressure measurements. Attempts to measure film or tape stress and strain, however, have not been completely successful. The hostile environment of flight is dynamic and includes a variety of heat transfer mechanisms which alter the balloon and sensor temperatures. The films are so thin that the presence of a sensor usually results in a localized stiffening of the film which alters the measurement. As a result, even large gage lengths of very stiff materials such as polyester films are used with only limited success. There is need for a strain gage suitable for flight which can verify or negate the assumptions contained in thin film balloon stress prediction analyses. In addition, need has been established for an analytic model of the thin polyethylene film used in fabrication of scientific balloon platforms. A description of the mechanical response of balloon materials is necessary if stresses are to be deduced from in-flight strain measurements. This invention relates to a unique sensor capable of monitoring the strain in the wall of a typical balloon when exposed to the hostile environment of ascent and float. Its use will facilitate prediction of balloon stresses for particular flights, and design and testing of balloon materials in general.
It is therefore an object of the present invention to provide a strain transducer system which is suitable for in-flight measurement of balloon film strain because of its lightweight, linear response to strain, and mechanical and electrical compatibility with existing balloon systems.
A further object of this invention is to provide a strain transducer with an annular configuration that is insensitive to the transverse forces and thermal distortions experienced by balloons during flight.
Another object of this invention is to provide a strain transducer with a low effective modulus of elasticity that is particularly sensitive to the relatively small longitudinal forces and large deformations suffered by balloons in flight.
A still further object of this invention is to provide a device that can function in ranges from relatively warm temperatures (+25.degree. C.) of an afternoon launch to the lowest temperatures (-80.degree. C.) found during ascent through the tropopause.
An additional object of the present invention is to provide a strain transducer system with attachment tabs that minimize the local discontinuities in film stress caused by stiffness at the points where the transducer is fitted to the thin film balloon surface.